The present invention relates to a new process for removing low-boiling by-products from a diurethane prepared by a phosgene-free process.
Diurethanes based on simple diisocyanates and low-boiling alcohols or phenols can be thermally decomposed to produce basic diisocyanates. One of the known methods for producing diisocyanates is preparation of a diurethane without the use of phosgene followed by thermal decomposition of that diurethane. Known methods for the phosgene-free preparation of diurethane include the reaction of a diamine with urea and alcohol (See, e.g., EP 18,586; EP 27,940; EP 27,953; EP 126,299; EP 126,300; EP 355,443; EP 568,782; and EP 566,925), reaction of a diamine with a carbamate and optionally an alcohol (EP 18,88; EP 27,952) and a reaction of a diamine with a carbonate (EP 323,514). The diurethanes prepared by any of these known methods contain a high-boiling fraction (e.g. oligoureas or polyureas) and impurities (derivatives of carbonic acid) which boil at a lower temperature than the diurethanes. To isolate the diurethanes, common separating techniques are generally applied.
EP 18,586 teaches that purification of the diurethane takes place after filtering off solids, e.g. by distilling off the excess alcohol and/or solvent and also the carbamate either formed as a by-product or used in excess. No data is given on the purity of the products obtained. The disadvantage of this disclosed process is that high-boiling impurities remain in the product. For complete separation of by-products which boil at a lower temperature than the diurethane by means of distillation, the reaction mixture must be exposed to heat for a certain length of time. This heating may cause the formation of additional decomposition products.
Crystallization or precipitation processes are also suggested as other purification possibilities. However, these purification processes are less economically viable for an industrial process due to the more complicated process engineering required.
EP 323,514; EP 27,940; EP 27,952; and EP 27,953 each teach that the product diurethane is worked-up by distillation. After distilling off the lower boiling solvents, auxiliary agents, reactants or intermediates, the urethanes are generally obtained as the last fraction or as the distillation residue. Before working up the product by means of distillation, insoluble components (e.g. insoluble catalysts) may be filtered off, if necessary. The disadvantage of these processes is that decomposition phenomena may be triggered in the products due to exposure to heat. This takes place in particular in the case of high-boiling diurethanes. Furthermore, the desired products may be contaminated by partial decomposition of the by-products during distillation.
EP 355,443 and EP 568,782 teach working-up of the diurethane containing mixture to remove unreacted alcohol by distillation. Then, N-unsubstituted carbamate and dialkyl carbonate are separated in a thin-film evaporator. The diurethane is obtained as the last distillation fraction.
In the process described in each of EP 126,299 and EP 126,300, a solvent (alcohol) is distilled out of the reaction mixture. In a second stage, the remainder of the solvent and derivatives of carbonic acid, dialkyl carbonates and/or alkyl carbamates originating from the preparation procedure are removed in a stripping column using an inert gas. Temperatures of up to 200.degree. C. are used, so decomposition reactions may occur. In an industrial process, additional expense is incurred due to the need to purify the inert gas before discharging that inert gas to the environment. The produced purified in this manner may still contain oligo-urea/polyurethanes which are removed as the distillation residue in a further distillation step, e.g. in a thin-layer evaporator. Temperatures of up to 300.degree. C. may be used.
EP 566,925 teaches separation of the alcohol, dialkyl carbonate and alkyl carbamate by reducing the pressure from the level using during the preparation stage (12 bar) to a pressure of 50 mbar. This process is very complicated from an engineering perspective. Further, complete elimination of the dialkyl carbonate or alkyl carbamate is not achieved. We have found that the product treated in accordance with this known procedure still contains up to 5% of these by-products which are subsequently found in the isocyanate produced by thermal decomposition of the urethane.